Patent GB-1376246-A (GB)
Title: Determining the oxygen content of blood
IPC: G01N21/31
Filed: 1972-01-25
Abstract: 1376246 Measurement of oxygen content of blood SIEMENS AG 25 Jan 1972 [23 March 1971] 3525/72 Heading G1A The invention relates to the photo-electric measurement of the oxygen content of blood and to a calibration arrangement to render the measurements independent of the haemoglobin content.  Apparatus. In the arrangement shown, a blood sample is contained in dish 1, and selenuim photocells 4', 5 and 12 receive light via filters, respectively: through dish 1 at a wavelength of 805 nm (isosbestic wavelength); through dish 1 at a wavelength of 650 nm; and through air or reference liquid, e.g. blood plasma, at 805 nm. The photocells are each connected in differential amplifier circuits 6-8; 7-9 and 13-14 to produce voltages representative of the received intensities. A switching arrangement 15-19 connects these voltages, together with a reference voltage via resistor 10, in a repetitive sequence to amplifier 20 and logarithmic circuit 28, 29, 30. The calibration circuit is shown as differential amplifier 35 with peripheral resistors and the potentiometers used in calibration. The switching of switches 31, 37 is such that the output of terminals 15, 16 are connected to resistor 32 and terminal 38, that from 17 to potentiometer 33 and terminal 39, and that from 18 to potentiometer 34 and terminal 38. Switches 54, 55 and 63, 64 are provided in differencing circuits 43, 44 to connect the outputs from terminals 16 and 17 to circuit 43 and the outputs from 16 and 18 to circuit 44. In this way circuit 43 produces a signal representing the difference between the intensities at 805 nm and at 650 nm through the dish 1, giving the absolute oxygen content of the blood sample, while circuit 44 produces the difference in the intensities both at 805 nm through and by-passing the dish 1, giving the haemoglobin content of the sample. The circuit 45 takes the quotient of these signals to provide the relative oxygen saturation indicated on e.g. voltmeter 46. Calibration procedure This involves firstly adjustment of the zero point to bring the linear curve of the output of logarithmic amplifier 28 for 650 nm against the Hb value of reduced blood samples into coincidence with the curve at 805 nm, so as to make measurements independent of the Hb value for any zero oxygen content samples to be tested. This is carried out by using two different Hb value reduced samples in alternate successive steps. Amplifier 35 gain is varied by potentiometer 33 to give zero output from difference former 43 for one sample, and a reference potential is varied by potentiometer 40 for the other sample. The output of difference former 44 is zeroed using only water or blood plasma in cell 1 by adjustment of potentiometer 34. The maximum signal adjustment is by means of potentiometer 65 using an oxygen saturated sample. Modifications and alternatives. Reflected light measurements may be used. Instead of the switching arrangements (which may utilize field effect transistors) duplicated circuitry may be used. Amplifier 35 and associiated resistors may be replaced by a T-network. In calibration of zero values, suitable coloured filters may be used instead of actual blood samples.